The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is defined as 299792458 metres per second (approximately 300000 km/s, or 186000 mi/s) Wikipedia. (as of 2021-01-26)

It is generally assumed that fundamental constants such as c have the same value throughout spacetime, meaning that they do not depend on location and do not vary with time. However, it has been suggested in various theories that the speed of light may have changed over time. No conclusive evidence for such changes has been found, but they remain the subject of ongoing research. Wikipedia. (as of 2021-01-26)

The Planck constant, or Planck's constant, is the quantum of electromagnetic action that relates a photon's energy to its frequency. The Planck constant multiplied by a photon's frequency is equal to a photon's energy. The Planck constant is a fundamental physical constant denoted as h. Wikipedia. (as of 2021-01-26)

The Planck constant is defined to have the exact value h= 6.62607015×10−34 J-s in SI units. Wikipedia. (as of 2021-01-26)

Note that physics constants are "defined" as such and such a value. They are assumed to not have changed over time. There appears to be evidence that such is not the case but that is not the topic of discussion here.

However, if, as some researchers have suggested (via data measurements, etc.), the speed of light was much faster in the past (e.g., exponentially greater) than certain other assumptions change.

For example, a galaxy that is claimed to be 5 billion light years away is assumed to be 5 billion years old. That is, if the speed if light never changes.

However, if the speed of light were exponentially faster in the past, then those 5 billion years might collapse to, say, 50,000 years or even 5,000 years.

To many, this would be unacceptable, so the assumption that the speed of light should never change must be maintained.

In the 1960's the speed of light was redefined to be based on certain atomic measurements. These measurements are themselves based on the speed of light.

So, using circular reasoning, the speed of light is defined in terms of the speed of light such that it never changes. Somewhat like the expression of a "rubber ruler" - it adapts to whatever it is that one is measuring.